As the reaction product of subducted water and the iron core, FeO2 with more oxygen than hematite (Fe2O3) has been recently recognized as an important component in the D” layer just above the Earth's core-mantle boundary. Here, we report a new oxygen-excess phase (Mg, Fe)2O3+δ (0 < δ < 1, denoted as “OE-phase”). It forms at pressures greater than 40gigapascals when (Mg, Fe)-bearing hydrous materials are heated over 1,500 kelvin. The OE-phase is fully recoverable to ambient conditions for ex-situ investigation using transmission electron microscopy, which indicates that the OE-phase contains ferric iron (Fe3+) as in Fe2O3 but holds excess oxygen through interactions between oxygen atoms. The new OE-phase provides strong evidence that H2O has extraordinary oxidation power at high pressure. Unlike the formation of pyrite-type FeO2Hx which usually requires saturated water, the OE-phase can be formed with under-saturated water at mid-mantle conditions, and is expected to be more ubiquitous at depths greater than 1,000 km in Earth's mantle. The emergence of oxygen-excess reservoirs out of primordial and subducted (Mg, Fe)-bearing hydrous materials may revise our view on the deep-mantle redox chemistry.
Knowledge of the stability of carbonate minerals at high pressure is essential to better understand carbon cycle deep inside the Earth. The evolution of Raman modes of carbonates with increasing pressure can straightforwardly illustrate lattice softening and stiffening. Here, we reported Raman modes of natural magnesite MgCO 3 up to 75 GPa at room temperature using helium as a pressure-transmitting medium (PTM). Our Raman spectra of MgCO 3 showed the splitting of T and ν 4 modes initiates at approximate 30 and 50 GPa, respectively, which could be associated with its lattice distortions The MgCO 3 structure was referred to as MgCO 3-Ⅰb at 30-50 GPa and as MgCO 3-Ⅰc at 50-75 GPa. Intriguingly, at 75.4 GPa some new vibrational signatures appeared around 250-350 and ~800 cm-1. The emergence of those Raman bands in MgCO 3 under relatively hydrostatic conditions is consistent with the onset pressure of structural transition to MgCO 3-Ⅱ reported by theoretical predictions and high pressure-temperature experiments. This study suggests that hydrostatic conditions may significantly affect the structural evolution of MgCO 3 with increasing pressure, which shall be considered for modeling the carbon cycle in the Earth's lower mantle.
Phase H (MgSiO 4 H 2 ), one of the dense hydrous magnesium silicates (DHMSs), is supposed to be vital to transporting water into the lower mantle. Here the crystal structure, elasticity and Raman vibrational properties of the two possible structures of phase H with Pm and P2/m symmetry under high pressures are evaluated by first-principles simulations. The cell parameters, elastic and Raman vibrational properties of the Pm symmetry become the same as the P2/m symmetry at ∼ 30 GPa. The symmetrization of hydrogen bonds of the Pm symmetry at ∼ 30 GPa results in this structural transformation from Pm to P2/m. Seismic wave velocities of phase H are calculated in a range from 0 GPa to 100 GPa and the results testify the existence and stability of phase H in the lower mantle. The azimuthal anisotropies for phase H are A P0 = 14.7%, A S0 = 21.2% (P2/m symmetry) and A P0 = 16.4%, A S0 = 27.1% (Pm symmetry) at 0 GPa, and increase to A P30 = 17.9%, A S30 = 40.0% (P2/m symmetry) and A P30 = 19.2%, A S30 = 37.8% (Pm symmetry) at 30 GPa. The maximum V P direction for phase H is [101] and the minimum direction is [110]. The anisotropic results of seismic wave velocities imply that phase H might be a source of seismic anisotropy in the lower mantle. Furthermore, Raman vibrational modes are analyzed to figure out the effect of symmetrization of hydrogen bonds on Raman vibrational pattern and the dependence of Raman spectrum on pressure. Our results may lead to an in-depth understanding of the stability of phase H in the mantle.
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